I want to make the two story 40 instead of 34. Is there anything I should know when adding onto the length.
Adding length is a piece of cake. :) You extend the foundation, add more joists and floor, add more walls down the sides, and make it longer. No need to re-engineer things like joists and rafters that go across the width. Increasing the width would be a whole new kettle of fish.
As Don said -- same spacings as the rest. No problem.
Check out Jimmy Cason's home, he extended it to 40 feet. As you can see in the pictures, you just make it longer.